Manufacture of steel in the modern BOF furnace is a highly automated and efficient method and has largely replaced manufacture of steel in the open hearth furnace.
As background material and description of a presently-used method of BOF operation, the furnace comprises a huge upright vessel 10 which is open at the top 11. The vessel has a steel shell 12 which is interiorly lined with fire brick 14. Trunnions 15 extend outwardly from opposite sides of the steel shell and are contained in bearings (not shown) which are mounted on a stationary part of the superstructure. The trunnions and bearings provide for tilting of the vessel about a horizontal axis. Most vessels are more than 32 feet in height and therefore represent a considerable mass to tilt. The vessels are moved about the trunnion pivots by motors controlled from the pulpit adjacent to the furnace.
As seen in FIG. 1, in order to charge the vessel 10, the same is tilted to the dotted line position A and steel scrap is dumped from a scrap car 16 into the vessel through its open end 11. The car 16 has wheels 17 which roll along rails to and away from position adjacent to the vessel. The car has a pivot support 18 so that it may be tilted to the dotted line position so as to dump its load into the vessel. In some cases instead of a wheeled car, a crane-supported scrap vessel is used.
After dumping its load, the car 16 is moved along the rails to a position removed from the vessel and wherein it may receive another load of scrap. A metal ladle 20 has trunnions 21 supported within crane hooks 22. The ladle is of substantial capacity and may contain about 150 tons of molton pig iron. After the scrap car has been removed, the ladle 20 is moved to the vessel 10 and tilted to pour its contents into the latter. Thereafter the ladle 20 is moved away from the vessel to a position wherein it may receive another charge of molten metal. Normally the charge into the vessel 10 consists of about 30% steel scrap and 70% molten pig iron, so that the metal charge into the vessel 10 is in the neighborhood of about 200 tons.
After the vessel 10 has received its charge, it is returned to upright position and the lance 26 is lowered into the vessel; a flow of oxygen is started and the steelmaking process is under way. Within seconds after the oxygen is turned on, it is ignited by the hot metal and reaction with the impurities of the charge commences. At this point in the process, the prescribed weight of fluxes are added to the vessel. Under normal operating conditions, the time elapsed from the charging of scrap to the start of the oxygen blow averages less than three minutes.
When the blow is completed, as determined by the furnace operator utilizing the results of calculations by a computer, the lance 26 is withdrawn and the vessel is tilted to a horizontal position toward the charging aisle, as shown by the dot-dash lines B. A temperature reading is secured with an immersion-type thermocouple, and a sample of the steel is obtained and sent to a nearby chemical laboratory where a vacuum spectrometer, within a few minutes, determines the individual content of chemical elements in the steel, including carbon, phosphorus and sulphur.
If the temperature and chemical content of the steel are correct, the vessel 10 is then tilted in the opposite direction shown by dot-dash lines C in FIG. 1 and the molten steel will drain through the tap hole 27 and into the teeming ladle 28. The slag floats on top of the molten steel, as shown at S in FIG. 4, and will not flow through the tap hole until substantially all the steel has been removed from the vessel, and when slag does appear at the tap hole the vessel 10 is immediately moved toward upright position.
After conclusion of tapping, the vessel 10 is tilted in a direction toward the charging aisle and beyond the position B to invert it and dump the slag remaining in the vessel into slag ports 29. From this inverted position, the empty vessel is returned to its charging position A to receive a charge for the next heat. All of the foregoing, from charge to subsequent charge, is accomplished in a matter of about 25 to 28 minutes and it will therefore be appreciated that the BOF process is highly efficient and economical, especially when compared with the open hearth process.
In actual practice, it has been noted that the vessel lining has a tendency to deteriorate in various areas thereof, and this may be due to various factors. For example, pure oxygen issues from the lance nozzle at a pressure that is normally held between 140 and 180 pounds per square inch. The action of the oxygen jet is partly chemical and partly physical. Striking the liquid bath, the oxygen immediately starts reactions leading to the formation of iron oxide, part of which disperses rapidly throughout the bath. Carbon monoxide is evolved, which gives rise to a vigorous boiling action and accelerates the refining metalergical reactions. Other factors also contribute to deterioration of the vessel lining, and such deterioration is particularly prevalent in the trunnion area, as indicated at 30 in FIG. 3 and the tap hole area, as indicated at 31 in FIG. 2.
If the lining deterioration is not corrected, the furnace will soon reach a point where it must be taken out of service for relining, and downtime for this is usually on the order of three to five days, assuming the lining material is immediately available.
A prior art process for correcting lining deterioration is shown in FIG. 2. This process is used after the slag has been dumped from the vessel. In such use, the vessel is moved from its inverted position to the horizontal position shown in FIG. 2. A tractor 35 carrying a horizontally disposed coating lance 36 is moved so that the latter enters the vessel 10 through its mouth. A refractory coating, in fluid form, is forced through the lance 36 and issues from the lance nozzle 37 and is deposited on the deteriorated areas of the lining, the lance being rotatable about its longitudinal axis to deposit the coating at any portion of the interior of the vessel.
Although this prior art process has produced acceptable results, it has added considerable time to the steelmaking cycle each time it is used. For example, the vessel 10 must be held at the horizontal position shown in FIG. 2, instead of moving uninterrupted from its inverted position to its charging position A. The tractor 35 must be driven to position adjacent to the mouth of the vessel and this is sometimes done with difficulty, considering the cluttered area adjacent to the furnace. It has been observed that use of the prior art process for lining repair has added about 15 to 40 minutes to a steelmaking cycle and this seriously affected the economical advantages of the BOF system. However, considering the alternative of a three- to five-day downtime for relining, the industry had no choice.